The present application is directed to subject matter of German Patent Application No. 199 28 773.2, filed on Jun. 23, 1999, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to the hardening of edge layers of machine components. Objects for which utilization is practical and useful are components that are heavily exposed to wear and fatigue that are produced from precipitation-hardenable materials because of the high demands in material strength with, at the same time, high toughness. The invention may be used particularly advantageously for increasing the wear resistance of components made from stainless, precipitation-hardenable martensic steel, such as, e.g., turbine buckets, pump shafts, highly stressed bolts in aeronautics, components in shipbuilding, or special tools. An additional area of use is components exposed to wear made from high duty martensite hardening (Maraging) steel that cannot be used in a fully hardened state when high demands of toughness are present.
2. Discussion of Background Information
During use, edge zones of components placed under wear and fatigue stresses are exposed to significantly different stress than the core of the component. This fact is known to be considered in producing in the edge zone a harder, more wear or fatigue resistant structure using thermal, physical, chemical, mechanical, thermodynamical, or thermomechanical procedures as compared to the core, whose structure is adjusted such that it primarily meets the present demands in hardening and toughness.
This background of the invention shall be explained in greater detail using a characteristic component, prototypically chosen, without limiting its general purpose. Rotating blades of low-pressure stages in steam turbines are exposed during their use to extremely high pseudostatic (centrifugal forces, blade torsion), cyclic stresses (periodic exposure to steam pressure, blade oscillations), and tribologic (impingement) loading. In particular, the constant impact of condensed water droplets leads to an eroding wear in the area of the leading edge of the blade. Martensite-hardened 13% steels are able to meet these complex demands. Here, the blade material is used in the hardened, tempered state (meeting the requirements in toughness, stress corrosion resistance, corrosion fatigue resistance, sufficient static and cyclic stress resistance; hardness about 250-350 Vickers hardness numbers (VHN)) and the area of the leading edge of the blade is short-time hardened, e.g., via flame, induction, or laser hardening (very high resistance to wear by impingement, 390-680 HV). Increasing requirements in static and cyclic working stress as well as resistance to stress corrosion or corrosion fatigue have lately lead to the use of non-corroding precipitation-hardenable martensitic steel. In relation to tempered steel, they do not receive the biggest part of the increase in hardening and toughness by the formation of martensite but by a controlled precipitation hardening.
For this purpose, the steel contains 10-20 wt-% chrome and 2-11 wt-% nickel, usually copper (1-5 wt-%), and aluminum, titanium, or niobium as a precipitation former. In turbine constructions, a typical representative of this type of steel is the steel X5CrNiCuNb16-4. The heat treatment usually contains at least one solution annealing at 1030-1080xc2x0 C. (duration of approximately 1 h.) and the precipitation treatment per se in the temperature range between 480xc2x0 C. and 620xc2x0 C. (duration 1-4 hs.) The achievable mechanical characteristics, hardening, yielding stress Rp0.2 and tensile strength Rm, reach their maximum at the lower limit of conventionally possible tempering temperature of 480xc2x0 C. and diminish drastically in increasing aging temperature (see Drawing 1). For instance, in the temperature range of 480-620xc2x0 C., the hardening drops from 425 HV to 285 HV, the yielding level from 1170 to 750 MPa, and the tensile strength from 1310 to 930 MPa. Due to the required toughness levels, resistance against stress corrosion, and corrosion fatigue, the tempering temperature must be chosen that high that the 0.2% yielding level remains below 1040 and the tensile strength below 1000 MPa. This means that the low range of the possible tempering temperature producing the high velocities cannot be used (see Drawing 1).
Therefore, the shortcoming of this conventional heat treatment process lies in the resistance to wear from impingement being too low. This is based on the hardening of 340-370 HV being too low near the surface.
It is known that the surface hardening of precipitation-hardenable steel can be increased by plasma-nitriding up to about 1000 HV [e.g., brochure of the company Bxc3x6hler Edelstahl GmbH (Kapfenberg, Austria) about the steel N700.] The shortcoming of this process includes that no improved resistance to impingement is achieved here, either. This shortcoming is based, e.g., on the fact that the achievable depth of nitriding of about 0.15 mm is much too low.
Other processes for tempering edge layers are not suitable either, since they affect the necessary aging treatment impermissibly or the achievable increase in hardness or the depth of the hardness is too low.
For improving the condition of the material itself, a process has become known in which a structure with a higher 0.2% yielding tension and tensile strength is achieved by coupling a short-time tempering with a conventional tempering treatment [see E. E. Denhard, Jr.: xe2x80x9cPrecipitation-hardenable stainless steel method and product,xe2x80x9d U.S. Pat. No. 3,660,176.] For this purpose, the entire partially-finished product is exposed to a thorough short-time heating in the temperature range between 816xc2x0 C. and 1149xc2x0 C. of the solution annealing treatment within a time frame of 1 to 15 seconds by a direct flow of current and rapidly cooled. Then a conventional tempering treatment occurs in the conventionally used temperature range. It is thus possible, with a solutions annealing temperature of 1149xc2x0 C., a solution annealing time of 2 seconds, a tempering temperature of 482xc2x0 C., and a tempering time of 1 hour, to raise the 0.2% yielding level from 1328 MPa to 1695 MPa and the tensile strength from 1378 MPa to 1700 MPa. The achieved hardness is not listed.
The shortcoming of this process is that it is not suitable for being used in components of complex forms such as turbine buckets. This shortcoming is caused by the heating measures being used, such as conductive or inductive heating, depending on geometric relationships.
Another essential shortcoming is the fact that the tougness and endurance range and, in particular, the resistance against stress corrosion and corrosion fatigue of a turbine bucket treated in this manner would be too low. The reason for this lies in the hardness of the interior of the blade being much too strong. If the turbine bucket were to be tempered at higher temperatures, however, the hardness in the area of the leading edge of the bucket would be too low. With this process for improving the state of the material itself, it is not possible to simultaneously accommodate the different requirements that are placed upon the edge layer and the core of the component.
Another shortcoming lies in the fact that a conventional performance of tempering hardening cannot utilize the capacity for hardening in the state of the short-time solution annealing completely. This is caused by two facts: first, higher hardening states of structure that contain the entire cross-section of the component cannot be used due to low toughness, and second, new metal-physical degrees of freedom that offer short-time solution annealing for subsequent tempering hardening are not known.
The object of the invention is to provide a new and effective heat treatment process that allows components of precipitation-hardenable materials to be provided with considerably better wear-resistant edge layers without having to accept a worsening of the remaining mechanical properties of the component.
The object of the invention is to provide a heat treatment process that allows a stronger hardness of the edge layers up to a sufficient depth with sufficient toughness to be achieved, depending on the tribological loading, independent of the structure and the mechanical characteristics of the component core, and without influencing them, which can also be used in components of complicated shapes and in which the tempering temperature better utilizes the hardening capabilities of the short-time solution annealing.
This object is attained according to the invention with a process for creating wear-resistant edge layers in precipitation-hardenable materials as described hereinbelow.
In the process of the invention for producing wear-resistant edge layers in precipitation-hardenable materials by means of a short-time solution annealing and a subsequent aging heat treatment a component that was conventionally solution annealed at a temperature Tcsa1 and subsequently subjected to a conventional aging heat treatment at a temperature Tcpa1 is subjected to another short-time solution annealing affecting only the edge layer of the component at a temperature Tssa greater than Tcsa1 and a duration of the short-time solution annealing xcex94tssa less than 12 s. Subsequently another aging heat treatment is performed that evenly includes the interior of the component and its edge layer at a temperature Tspa less than Tcpa1. In one aspect of this process, the edge layer of the component up to a depth tH that corresponds to the desired hardening depth is solution annealed by means of a short-time energy impact originating in the surface of the component; the short-time energy impact originating in the surface of the component is achieved by means of a high-energy edge surface heating process; the heating speed (xcex94T/xcex94t)ssh reaches values of 102 K/sxe2x89xa6(xcex94T/xcex94t)sshxe2x89xa6104 K/s; the temperature gradient (xcex94T/xcex94r)ssh is selected in the range of 13 K/mmxe2x89xa6(xcex94T/xcex94r)sshxe2x89xa61000 K/mm; Tcsa2+50 Kxe2x89xa6Tmax ssaxe2x89xa6Tcsa2+400 K applies as the peak temperature of the short-time solution annealing treatment Tmax ssa, with Tcsa2 being the conventional solution annealing temperature of the corresponding material; the duration of the short-time solution annealing xcex94tssa in the temperature range in which a noticeable dissolution of the precipitations occurs lies in the range of 10xe2x88x921 sxe2x89xa6xcex94tssaxe2x89xa612 s; the cooling speed (xcex94T/xcex94t)ssc attains maximum values in the cooling cycle of 5 K/sxe2x89xa6(xcex94T/xcex94t)sscxe2x89xa6104 K/s; the aging heat treatment is performed with a longer duration as compared to the short-time solution annealing xcex94tspa, xcex94tspa greater than xcex94tssh and with a significantly lower temperature gradient (xcex94T/xcex94r)spa, (xcex94T/xcex94r)spa less than  less than (xcex94T/xcex94r)ssh; Tspaxe2x89xa6Tcpa2xe2x89xa6Tspa+80 K applies as the temperature Tspa of the aging heat treatment, with Tcpa2 being the lower limit of the conventional temperature range for the aging heat treatment; and the duration of the aging heat treatment xcex94tspa is one and a half to sixteen times as long as the holding time xcex94tcpa2 of a conventional aging heat treatment. The process is based on functional optimizing by separately adjusting the structure in the component core and the edge layer. Here, the state of the structure in the component core as well as the core strength and toughness resulting therefrom are adjusted by means of a previous conventional heat treatment. Then the solution tempering of the edge layers occurs in a highly inhomogeneous temperature field, followed by an annealing process of the entire component, modified according to the invention, in a homogeneous or almost homogeneous temperature field. The requirements in depth, width, position, and progression of the wear protection zone, resulting from the analysis of the tribological loading and/or cyclic distribution of stress, are equivalent to the desired geometry of the solution annealing zone. The solution annealing zone is created by an edge layer heating process with a sufficient power density. The depth tH of the desired solution annealing zone is adjusted by the locally absorbing energy density and the local energy impact duration. The energy density and the energy impact duration also control the resulting heating speed (xcex94T/xcex94t)ssh and the temperature gradients (xcex94T/xcex94r)ssh.
The selection of the two parameters as well as the duration xcex94tssa and the peak temperature Tmax ssa of the short-time solution annealing within the predetermined data range ensures a sufficiently quick dissolution of the precipitation without the danger of coarser grain. Dependent on the peak temperature Tmax ssa and the original structure and the chemical composition of the component the cooling speed (xcex94T/xcex94t)ssc according to the invention prevents coarser grain during the cooling process and an uncontrolled precipitation hardening. The determination of the unusually high amount for the maximal temperature peak Tmax ssa utilizes the knowledge that the hardness of the edge layer being the main parameter determining the wear resistance in coordinating wear types increases with rising temperature or decreases only little. This way even in greater depths a solution state of precipitations can be achieved that ensures a stronger hardening depth or a slower hardening decrease. A specific embodiment of the invention for the classes of martensitic precipitation-hardenable steels comprises performing the edge surface processing of precipitation-hardenable steels with carbon contents of 0.03 to 0.08 weight-%, chrome contents of 10 to 19 weight-%, nickel contents of 3.0 to 11.0 weight-%, copper contents of 1.0 to 5.0 weight-%, and niobium contents of 0.15 to 0.45 weight-% in such a way that the depth tH of the solution annealed edge layer is 0.1 mmxe2x89xa6tHxe2x89xa67 mm; 1080xc2x0 C.xe2x89xa6Tmax ssaxe2x89xa61350xc2x0 C. applies as the peak temperature Tmax ssa of the short-time solution annealing; the temperature Tspa of the aging heat treatment is selected in the range of 445xc2x0 C.xe2x89xa6Tspaxe2x89xa6500xc2x0 C.; and the duration of the aging heat treatment xcex94tspa is set in the range of 1 hxe2x89xa6xcex94tspaxe2x89xa68 h. By choosing the parameters according to the invention for the peak temperature Tmax ssa, the temperature Tspa and the time xcex94tspa, a significantly stronger edge layer hardness is achieved.
The embodiment of the process of the present invention wherein after the short-time solution annealing treatment and before the aging heat treatment, a mechanical deformation of the edge layer is performed, is advantageous in that it can improve the state of the internal stress of the precipitation-hardened edge layer and in that a larger amount of nucleation sites for the formation of fine precipitations is available.
It is particularly advantageous when the process steps of short-time precipitation annealing, mechanical deformation, and precipitation heat treatment are combined in the processing of partially finished products where these partially finished products receive their final form by means of a deformation and/or where the short-time solution annealing treatment, the deformation, and the aging heat treatment take place in a continuous process.
The embodiment wherein the mechanical deformation of the edge layer is performed by means of shot peening with steel balls can be used particularly advantageously for the optimizing of the edge layer characteristics of very complicatedly shaped or very locally treated components such as turbine buckets.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.